This invention relates to a fluid seal, and more particularly, to a gasket adapted to be mounted between an engine head and block of an internal combustion engine, such as a diesel engine.
The current energy crisis has given rise to an array of lightweight, high output, gasoline and diesel engines, both aspirated and turbocharged. One result of this shift to lighterweight engines, for a number of reasons, is an increased potential for the abrasive-erosion of gaskets or abrasive gasket erosion. Abrasive gasket erosion occurs when a gasket is placed between and in contact with the engine block and cylinder head and then subjected to a substantial compressive load or stress and high frequency, cyclic impulse forces and vibrations generated by the engine. These forces and vibrations tend to impart very slight relative movement between confronting engine surfaces and the gasket.
The search for lighter-weight engines has also resulted in fewer headbolts and less efficient bolting patterns. In each case hold down torque is reduced below the optimum value. Typically, the gasket must withstand combustion pressures of 7,000 kilopascals (kPa)* in a naturally aspirated spark-ignition engine and 19,000 kilopascals or higher in a turbocharged diesel engine. Because of these high stresses, lighterweight castings and lighterweight, less rigid materials when used to form cylinder heads and engine blocks tend to result in greater relative motion between the two bolted together components and the sealing gasket. FNT *1 pascal=1 newton/meter.sup.2 =1.450377.times.10.sup.-4 psi
A number of possible solutions have been proposed to eliminate or reduce the potential for abrasive-erosion, many of which are impractical. For example, softer smoother materials which are less likely to erode the gasket could be used. Also, increasing surface hardness by shot-peening or cold-working decreases the tendency for the gasket to be eroded by the metal surfaces. This solution, however, requires, in some cases, that the entire engine assembly be redesigned which, in many cases, is very expensive if not totally impracticable. Another possibility would be to decrease the compressive loading at the bearing surfaces. Although this would reduce the forces tending to cause erosion, the sealing effect of the gasket would also be reduced, and even then that might not solve the problem because even very small loads are capable of producing damage by abrasive-erosion. Still another solution would be to lubricate the surfaces with high-viscosity, high-tenacity oils, greases or lubricants. Thus, while there are a variety of approaches that may be taken to reduce gasket erosion these approaches must be considered against their effects on the overall sealing capacity of the gasket.
Further, an important physical property of any gasket is good torque retention. In general, torque retention is associated with the amount of compressible material in a gasket and the compression set characteristics of the gasket material. Specifically, if the amount of compressible material is reduced, torque retention improves. Nevertheless, even when thin materials are used, there must be sufficient compressibility consistent with a good seal. Thus, a problem is presented in maintaining good torque retention while at the same time resisting the effects of abrasive-erosion through the application of erosion resistant materials. In other words, one cannot simply add materials to a gasket assembly without recognizing the potential for increased compression set and long term torque loss.